This invention relates to a fail-safe voltage-limiting circuit for an audio frequency overlay track circuit, which type of track circuit has found particular acceptance in the highway-crossing area and certain mass transit installations of the railroad signaling field.
Such overlay track circuits typically employ an AM or amplitude-modulated signal superimposed over the existing track circuits for the longer track sections and transmitted over the rails such that; in the presence of a train, the AM signal is shorted out by the shunting effect of the train wheels, and the absence or drop-out of the AM signal is therefore used to operate the track circuit relay. It has been found, in using the rails for communicating the AM signal between the ends of the track sections, that electrical noise can occur of sufficient magnitude such that the signal receiver operates as if the overlay signal is present which can be especially dangerous when, in the presence of a train, such noise can result in falsely indicating an unoccupied track section. Such noise can arise from various sources; for instance, in territory electrified for train propulsion, insulated joints, and impedance bonds are used to bridge the propulsion current across the offset rail joints resulting in an imbalance of the return-current which, in turn, causes EMI spikes on the order of 60 to 90 volts. Still more electrical noise is introduced to the rails by the propulsion energy itself which can typically be on the order of 10 kilovolts, 25 hertz source and which, due to the stopping and starting of a chopper-controlled vehicle, can result in electrical spikes of sufficient magnitude to be detectable by the receiver.
Additionally, in the railroad signaling industry where circuit components can have operating voltage levels ranging between 9.5 and 16 volts, it is advantageous that a fail-safe voltage-limiting circuit operate effectively within this range, as well, using a fixed arrangement of components, thereby facilitating maintenance and device-interchangeability.
Inherent in the design of circuits relating to railroad track circuits, is the need to provide fail-safe protection such that, failure of any component within the circuit results in an output corresponding to the most-restricted condition.